Lyme disease, caused by the spirochetal bacterium Borrelia burgdorferi, is the most prevalent arthropod-borne infection in the United States and has been classified as a re-emerging vector-borne infection due to a significant increase in the number of reported cases. Lyme disease is an important public health issue, particularly in endemic states where it contributes to significant rates of morbidity. The ability of B. burgdorferi to cause infection in mammals is dependent on its ability to alter gene expression rapidly in response to highly disparate environmental signals that it encounters upon transmission through a tick bite. Several genome-wide transcriptional analyses, comparing B. burgdorferi cultivated under conditions that mimic the tick vector or the mammalian host, revealed that several plasmid encoded genes on linear plasmid 54 (Ip54) are up-regulated under mammalian host-specific conditions. Sequence based secondary structure analysis of several of these differentially expressed ORFs revealed that BBA34 has an arginine-glycine-glutamic acid (RGD) motif at the N-terminal region and that this motif is probably surface-exposed. Moreover, based on sequence similarity, BBA34 is an orthologue of oligopeptide permease A (OppA5) protein but does not bind heptapeptides like other chromosomally encoded OppA orthologues and is up-regulated in B. burgdorferi upon transmission to mammalian host. The objective of this proposal is to determine if the RGD motif on BBA34 will facilitate interactions with mammalian cells presumably through direct interactions with integrins. The first specific aim is to determine the interactions of recombinant BBA34 proteins with various integrins either purified or expressed on mammalian cells. We will determine the role of RGD motif in binding to various integrins using full-length, N-terminal and C-terminal recombinant BBA34 proteins. The second specific aim is to determine the binding, infectivity and tissue dissemination characteristics of a bba34- mutant of B. burgdorferi in the murine model of Lyme disease. The third specific aim is to characterize the binding and infectivity of bba34-/bbk32- and bba34-/p66- double mutants of B. burgdorferi in the murine model of Lyme disease. We will determine the significance of the loss of multiple borrelial determinants on host-cell adherence. These studies will help to determine the host-pathogen interactions that are critical for the colonization of the mammalian host by B. burgdorferi.